1. Field of the Invention
This invention relates to an improved seal system designed for particular use in completion and production operations of oil and gas wells wherein the seal system greatly reduces the tendency and incidence of seal extrusion, seizure or sticking.
2. Description of the Prior Art
Movement of tubing within the sealng bores of packers, bridge plugs, and the like, during completion and production operations in high pressure, high temperature oil and gas wells has been a major cause of damage to seal systems and subsequent diminished performance and sealing integrity of such seal systems. Damage to the seal systems has become a greater problem during the present intensive search for new oil and gas reserves, wherein the drilling and subsequent completion is in deeper wells, involving exposure to extremely hostile high temperature, high pressure environments and wherein the well production may contain a significant amount of hydrogen sulfide, carbon dioxide and methane.
To overcome these conditions and thus successfully complete such a well, the seal system must have continuous sealing integrity. The seal system must be resistant not only to the damaging effects of the well environment (i.e., temperature, pressure, fluids) but also to the physical stresses imposed by or resulting from completion or workover techniques, i.e., unrestrained tubing movement, build up of corrosive products in the sealing annular area, and the like.
Typical prior art seals for use between nested subterranean tubular conduits are seals such as O-ring seals and T-seals, which respectively have circular and T-shaped cross sections. Seal stacks having chevron shaped cross-sections are also quite common. U.S. Pat. No. 3,799,204 discloses a subterranean safety valve using O-ring seals, T-seals and chevron seals. Seals having a triangular or delta shaped cross section have also been used between concentric tubular members. Back-up or bearing elements to prevent extrusion of elastomeric sealing elements have been used with T-seals, with chevron seals and with delta seals.
U.S. Pat. No. 2,862,563, illustrates a well packer assembly for packing the annular space between tubing in a well, wherein resilient annular packing elements are spaced about a tubular mandrel to form a seal in the annulus. U.S. Pat. No. 3,083,775 teaches the use of a formation packer wherein a plurality of resilient annular packing elements are spaced about a tubular mandrel and a plurality of folded metal plates which are set on a double traveling mandrel. U.S. Pat. No. 3,531,236 discloses a tubular sealing assembly having chevron sealing rings formed from a fluoroelastomer and asbestos, with fluorocarbon plastic ring adaptors at each end of the seal stack. U.S. Pat. No. 2,467,822 teaches the use of a rubber or similar packing material which is prevented from flowing through the opening between the packer body and the packing retainer or abutment surrounding the body.
The prior art also shows a number of generic seal systems which may have utility in the sealing of an annular well conduit. For example, U.S. Pat. No. 3,467,394 shows a packing means of a V-ring type wherein the packing arrangement comprises a polytetrafluoroethylene, commonly marketed under the trademark "TEFLON", with relatively rigid V-ring shaped spacer rings interposed between a plurality of the elastomer V-rings. Additionally, U.S. Pat. No. 4,050,701 shows ring seals obtained from a mixture of polyphenylene sulfide and polytetrafluoroethylene, for use in the fluid sealing of rotary and/or reciprocating shafts. Similarly, U.S. Pat. No. 3,626,337 discloes a packing ring for use in high temperature and pressure environments wherein a thermoplastic type composition, such as rubberized nylon, tetrafluoroethylene, polyesters, acrylics and the like, are laminated to form the final seal substance. Lastly, U.S. Pat. No. 3,799,454, discloses a coating composition containing polytetrafluoroethylene and polyphenylene sulfide for formation of a seal system.
Generally, the seal systems of the prior art have not been totally suitable for use in wells having high temperature and pressures and corrosive fluids, and have also been found to be deficient when exposed to even less severe environments.
To overcome these deficiencies it has been suggested that various new elastomeric type materials be used in the sealing systems. A report of the 52nd Annual Fall Technical Conference and Exhibition of the Society of Pertroleum Engineers, Oct. 9 through 12, 1977 discusses the testing of a number of the various new elastomeric type materials in relation to their use in packer seal systems. This Report No. SPE 6762 discloses the use of three particular elastomeric type materials which can be utilized in packer seal systems. These elastomeric materials are commonly sold under the trademarks of "TEFLON", a polymer of tetrafluoroethylene, "RYTON", a polymer of polyphenylene sulfide, and "KALREZ", a perfluoroelastomer.
Polytetrafluoroethylene, sold under the DuPont trademark "TEFLON", is a flexible fluoropolymer having a high degree of permanent set and cold flow. It also has a high resistance to corrosive chemicals and high temperatures. It can be used as virgin material or it may be filled with a suitable filler, i.e. glass particles.
Polyphenylene sulfide resin, sold under the trademark "RYTON" by Phillips Petroleum Company, is a thermoplastic resin characterized by a high thermal stability, good bearing properties excellent chemical resistance, and a high affinity for fillers. The resin may be the reaction product of p-dichlorobenzene and sodium sulfide in a polar solvent, as disclosed in U.S. Pat. No. 3,354,129.
The perfluoroelastomer, sold under the trademark "KALREZ" by DuPont, is a material characterized by high thermal stability and excellent chemical resistance. Exemplary of such perfluoroelastomers is the reaction product of perfluoro (3-phenoxpropylvinyl ether) and at least one fluorine-containing ethylenically unsaturated monomer, as fully disclosed in U.S. Pat. No. 3,682,872.
The above described elastomeric materials have been used in packer seal systems, but not with complete success. It has been found that seal systems which incorporate these elastomeric materials have a strong tendency to extrude and to adhere or stick to the conduit when the seal system must be retrieved from the well.
A commercially successful seal assembly made from "TEFLON", "RYTON" and "KALREZ" components is disclosed in U.S. Pat. No. 4,234,197. Although, the results achieved by the seal configurations disclosed in this patent were far better than prior art seals, the seals were still subject to failure due to extrusion when pressure differentials in excess of 20,000 psi were required to be sealed. In many wells being currently drilled, and produced today, pressures in excess of 20,000 psi can be encountered. There is, therefore, a need for a sealing arrangement between two nested conduits which will successfully withstand pressure differentials in excess of 20,000 psi without incurring severe extrusion of the sealing material or failure of the seal due to sticking of the sealing material whenever relative movement occurs between the two conduits.